Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. In recognition of these expenses, added emphasis has been placed on efficiencies associated with well completions and maintenance over the life of the well. Further, ever increasing well depths and sophisticated architecture are also employed for the sake of maximizing the efficiency and total hydrocarbon recovery from a given hydrocarbon reservoir. For example, vertical well depth may exceed 10,000 feet in order to reach deep reservoirs as well as to help ensure maximum vertical contact with the reservoir.
Of course, even maximum vertical contact with the reservoir is limited given that the reservoir, as with most any other geologic formation, is found in a layered fashion. For example, it would not be uncommon to find a reservoir that occupies about 100 feet vertically, but tens of thousands of feet horizontally. As a result, well architecture often includes a cased vertical well section that extends into an open-hole horizontal or lateral leg section. In fact, the main bore of the well may vertically traverse the reservoir with several lateral legs emerging therefrom and going in different horizontal directions through the reservoir.
Not only does each horizontal leg add to the amount of contact with the reservoir, each leg may add dramatically to the amount of contact. For example, even in the noted circumstance where the well may afford 100 feet of vertical contact to the main bore, each lateral leg may extend to 10,000 feet or more horizontally through the reservoir. Thus, where perhaps five such horizontal legs emerge from the main bore in this fashion, 50,000 feet of added contact surface with the reservoir is provided by way of these legs. In a case where each of the legs is about the same diameter as the main vertical bore, this means that about 500 times the amount of interface with the reservoir has been provided by way of the legs as compared to the main bore.
Of course, with tens of thousands of feet of added well space provided by horizontal legs as described above, tens of thousands of feet of added well maintenance and management is required. For example, production from the reservoir may change over time and contaminants, often water, may begin to be produced. However, it is unlikely that water will suddenly be produced from all regions of the horizontal legs simultaneously. Rather, water production is likely to emerge inconsistently at isolated locations of one horizontal leg or another. Nevertheless, because each horizontal leg emerges from the same main bore, water production at a single location of a single horizontal leg may adversely affect all of production operations. Stated another way, the dramatic increase in reservoir contact area afforded by the horizontal legs has also dramatically increased the likelihood of water production with the potential to affect production operations.
In order to address the possibility of water production from a single location adversely affecting all production operations in a well of multilateral architecture as described, each leg is generally compartmentalized into zonally isolated regions. For example, the architecture of a 10,000 foot leg may be zonally isolated into five separate 2,000 foot sections. In this way, each section may be monitored throughout production operations for water production. Thus, once sufficient water production is detected at one of the sections, production therefrom may be closed off or reduced, for example, by closing a sliding sleeve or valve actuated by surface control or through an intervention if necessary.
The ability to detect water production and independently close off production, section by section as noted above, generally involves the placement of a dedicated capacitance measurement tool within each section. Each tool may be provided with the ability to communicate with equipment at the oilfield surface. So, for example, where water production is detected by the tool within the third section of the second lateral leg, the operator at the surface of the oilfield is provided with such information and may take appropriate corrective action (e.g. to direct closing of a sliding sleeve in the section to stop production therefrom). As a result, production from all other sections of the second leg and all other legs may proceed unaffected by any water production from the now closed off section.
Unfortunately, the ability to detect true water production or an accurate “water cut” by a conventional capacitance measurement tool is limited. Specifically, the amount of water that is being produced as compared to the totality of the produced fluid may not be ascertained with a great degree of accuracy, particularly where the water cut exceeds about 25-30%. That is, once the water being produced exceeds 25-30% of the produced fluid, the readings obtained from a capacitance measurement tool will continue to show only a 25-30% cut, even though the actual cut may be 35-80% or more.
Often, the inability to detect the true water cut is not of major concern. For example, it may be desirable to close off all production from a section whenever water production exceeds about 10%. Thus, the inability to accurately ascertain a 55% cut, for example, is of no significance. However, this is not always the case as many times it is desirable to know the true water cut even if well over 30%.
In wells such as those described hereinabove where an extensive amount of interface with the reservoir takes place in through horizontal legs, the true cut value over 30% is likely to be of significance. In multilaterals such as these, water cut may exceed 30% but only in an intermittent fashion. That is, the horizontal architecture may serve to promote a phase separation between water and hydrocarbons such that surges of water emerge periodically. Thus, the detection of water may not actually be an indication that the entire section being monitored has transitioned to an overall state of high water production. Instead, it may merely be that a temporary surge of water is detected as a result of the well architecture. Of course, the significance of such a temporary surge is unknown if the operator is never made aware of whether the water cut from the surge is 30%, 100%, or some other value in between. As a result, the operator may be left to assume a 100% water cut whenever the cut detection is over 30%. This may in turn lead to prematurely and unnecessarily closing off the leg section. In fact, given that the well is multilateral in nature, this natural phase separation problem is likely to be repeated at several different sections of the well. Therefore, a dramatic reduction in the overall productivity and efficiency of the well may take place as a multitude of well sections are prematurely closed off due to the unavailability of true water cut data in excess of about 30%.